[compiler-rt] [compiler-rt][RISCV] Implement __riscv_ifunc_select (PR #85790)

[Piyou Chen via llvm-commits]

BeMg wrote:

1. Align with latest `sys_riscv_hwprobe`
2. Update `__riscv_ifunc_select`, `from __riscv_ifunc_select(char *)` to `__riscv_ifunc_select(unsigned long long, unsigned long long)`.
3. Remove the cpuinfo relate code and string process relate code
4. Use the bitset method to determine whether a set of extension is available for current environment.

https://github.com/llvm/llvm-project/pull/85790